Dequeue Operation in DSA C++ - Interactive Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to retrieve the front element of the deque.

DSA C++

deque<int> dq;
// Assume dq has some elements
int frontElement = dq.[1]();

Drag options to blanks, or click blank then click option'

Apop_front

Bfront

Cpop_back

Dback

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop_front() when you want to access the element without removing it.

Using back() instead of front() to get the front element.

The front() function returns the element at the front without removing it. pop_front() removes it.

2fill in blank

medium

Complete the code to remove the last element from the deque.

DSA C++

deque<int> dq;
// Assume dq has some elements
 dq.[1]();

Drag options to blanks, or click blank then click option'

Apop_back

Bpop_front

Cfront

Dback

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop_front() instead of pop_back() to remove the last element.

Using back() which only accesses the last element but does not remove it.

pop_back() removes the last element from the deque.

3fill in blank

hard

Fix the error in the code to correctly remove and print the front element of the deque.

DSA C++

deque<int> dq = {10, 20, 30};
int x = dq.[1]();
dq.pop_front();
cout << x << endl;

Drag options to blanks, or click blank then click option'

Apop_front

Bpop_back

Cback

Dfront

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop_front() to get the element which causes a compilation error.

Using back() instead of front() to get the front element.

front() returns the front element; pop_front() removes it. Using pop_front() to get the element causes error.

4fill in blank

hard

Fill both blanks to correctly remove and print the last element of the deque.

DSA C++

deque<int> dq = {5, 15, 25};
int y = dq.[1]();
dq.[2]();
cout << y << endl;

Drag options to blanks, or click blank then click option'

Aback

Bpop_front

Cpop_back

Dfront

Attempts:

3 left

💡 Hint

Common Mistakes

Using front() or pop_front() instead of back() and pop_back().

Removing the element before accessing it.

back() returns the last element; pop_back() removes it.

5fill in blank

hard

Fill all three blanks to remove the front element, add a new element at the back, and print the new back element.

DSA C++

deque<int> dq = {1, 2, 3};
dq.[1]();
dq.[2](4);
int z = dq.[3]();
cout << z << endl;

Drag options to blanks, or click blank then click option'

Apop_front

Bpush_back

Cback

Dpop_back

Attempts:

3 left

💡 Hint

Common Mistakes

Using pop_back() instead of pop_front() to remove front element.

Using front() instead of back() to get last element.

pop_front() removes the front element; push_back(4) adds 4 at the back; back() returns the last element.

Practice

1. What is the main difference between a deque and a regular queue in data structures?

easy

A. Deque allows insertion and deletion from both ends, queue only from front and rear respectively

B. Deque only allows insertion at the rear, queue allows insertion at both ends

C. Deque is always sorted, queue is not

D. Deque uses arrays, queue uses linked lists

Solution

1. Step 1: Understand queue operations

   A regular queue allows insertion at the rear and deletion from the front only.

2. Step 2: Understand deque operations

   A deque (double-ended queue) allows insertion and deletion at both front and rear ends.

3. Final Answer:

   Deque allows insertion and deletion from both ends, queue only from front and rear respectively -> Option A

4. Quick Check:

   Deque = double-ended queue [OK]

Hint: Deque means double-ended, so both ends allowed [OK]

Common Mistakes:

• Thinking deque only inserts at rear
• Confusing deque with sorted data structure
• Assuming deque uses only arrays

2. Which of the following is the correct syntax to insert an element 10 at the front of a std::deque<int> named d in C++?

easy

A. d.push(10);

B. d.push_front(10);

C. d.insert_front(10);

D. d.add_front(10);

Solution

1. Step 1: Recall deque insertion methods

   In C++, std::deque uses push_front() to insert at the front.

2. Step 2: Check method names

   Only push_front(10); is valid syntax for front insertion.

3. Final Answer:

   d.push_front(10); -> Option B

4. Quick Check:

   push_front() inserts at front [OK]

Hint: Use push_front() to add at front in deque [OK]

Common Mistakes:

• Using push() which is not a deque method
• Using insert_front() which doesn't exist
• Confusing with vector methods

3. What will be the output of the following C++ code snippet?

std::deque<int> d = {1, 2, 3};
d.push_front(0);
d.push_back(4);
d.pop_front();
for (int x : d) std::cout << x << " ";

medium

A. 1 2 3 4

B. 0 1 2 3 4

C. 1 2 3

D. 0 1 2 3

Solution

1. Step 1: Trace deque operations

   Initial deque: [1, 2, 3]
   After push_front(0): [0, 1, 2, 3]
   After push_back(4): [0, 1, 2, 3, 4]
   After pop_front(): removes 0, deque becomes [1, 2, 3, 4]

2. Step 2: Print elements

   Elements printed are 1 2 3 4 with spaces.

3. Final Answer:

   1 2 3 4 -> Option A

4. Quick Check:

   pop_front removes first element [OK]

Hint: pop_front removes first inserted element [OK]

Common Mistakes:

• Forgetting pop_front removes 0
• Confusing push_front and push_back order
• Printing without spaces

4. Identify the error in the following C++ code using std::deque:

std::deque<int> d;
d.push_back(5);
d.pop_front();
std::cout << d.front();

medium

A. deque must be initialized with size

B. push_back() syntax is incorrect

C. pop_front() called on empty deque causes undefined behavior

D. front() cannot be used after pop_front()

Solution

1. Step 1: Analyze deque state after operations

   Initially, deque is empty.
   push_back(5) adds 5: deque = [5]
   pop_front() removes 5: deque becomes empty again.

2. Step 2: Check front() usage

   Calling front() on empty deque is undefined behavior and may cause runtime error.

3. Final Answer:

   pop_front() called on empty deque causes undefined behavior -> Option C

4. Quick Check:

   front() on empty deque is invalid [OK]

Hint: Don't pop from empty deque before accessing front [OK]

Common Mistakes:

• Assuming front() returns 5 after pop_front
• Thinking push_back syntax is wrong
• Believing deque needs size at initialization

5. Given a std::deque<int> d = {10, 20, 30, 40, 50}, which sequence of operations will result in the deque containing 10 20 30 after all operations?

hard

A. d.pop_back(); d.pop_front();

B. d.pop_front(); d.pop_back();

C. d.pop_front(); d.pop_front();

D. d.pop_back(); d.pop_back();

Solution

1. Step 1: Start with initial deque

   Deque = [10, 20, 30, 40, 50]

2. Step 2: Apply operations for each option

   d.pop_back(); d.pop_back();: pop_back() removes 50 -> [10, 20, 30, 40], pop_back() removes 40 -> [10, 20, 30]
   d.pop_front(); d.pop_back();: pop_front() removes 10 -> [20, 30, 40, 50], pop_back() removes 50 -> [20, 30, 40]
   d.pop_front(); d.pop_front();: pop_front() twice removes 10 and 20 -> [30, 40, 50]
   d.pop_back(); d.pop_front();: pop_back() removes 50 -> [10, 20, 30, 40], pop_front() removes 10 -> [20, 30, 40]

3. Step 3: Compare results

   Only d.pop_back(); d.pop_back(); results in [10, 20, 30]. The other sequences result in different contents.

4. Final Answer:

   d.pop_back(); d.pop_back(); -> Option D

5. Quick Check:

   Two pop_back() removes the last two elements [OK]

Hint: pop_back twice removes last two elements [OK]

Common Mistakes:

• Removing wrong ends in wrong order
• Assuming pop_back twice keeps 20
• Confusing pop_front and pop_back effects